Preparation of textile preforms for brake discs in composite material

ABSTRACT

A fiber preform for manufacturing a brake disk out of composite material is itself manufactured by superposing and bonding together fiber layers. Structural fiber layers ( 10 ) are used to form at least a first preform portion that is to constitute the fiber reinforcement of the brake disk core, while the or each preform portion that is to constitute the fiber reinforcement of a friction portion of the brake disk is constituted by a felt ( 16, 18 ), at least in its portion adjacent to the friction face.

The present invention relates to manufacturing brake disks out ofcomposite material, and in particular to preparing fiber preforms forsuch manufacture. The field of application of the invention is moreparticularly that of brake disks made of thermostructural compositematerial. Thermostructural composite materials for brake disks aretypically carbon-carbon or “C—C” composites constituted by a reinforcingpreform of carbon fibers densified by a carbon matrix and optionallysubjected to final siliciding treatment. Other suitable compositematerials are ceramic matrix composites or “CMCs” constituted by areinforcing preform of refractory fibers (carbon or ceramic) densifiedby a ceramic matrix, e.g. carbon-silicon carbide or “C—SiC” composites.

The use of thermostructural composite materials, in particular C—Ccomposites, for making brake disks is well known, in particular for themulti-disk brakes of airplanes, and also for land vehicles, e.g. F1racing cars.

The usual technique for manufacturing such disks consists in makingannular fiber preforms and in densifying them with a matrix of carbonthat fills the pores of the preforms.

The preforms are usually made by superposing layers of fiber fabricwhich are bonded together, in particular by needling, so as to give thepreform the cohesion it requires to avoid any risk of a diskdelaminating while it is in use. The fiber fabric layers are typicallymultidirectional two-dimensional layers formed at least in part out ofcontinuous filaments, e.g. layers made by weaving or braiding orknitting threads made up of continuous or discontinuous fibers, orlayers made up of a plurality of sheets of unidirectional cablesdisposed in different directions and bonded together by light needling.Fiber webs or layers of felt can be added to provide discontinuousfibers that are easily taken by the needles during needling to provideZ-direction bonding between layers (i.e. transversely relative to thefaces of the layers). These fiber webs or felt layers also serve torecycle the fiber scrap that is produced when cutting the fiber fabriclayers as is necessary to obtain annular preforms.

The use of fiber webs or felts made from such scrap material isdescribed in particular in documents FR-A-2 626 294 and EP-A-0 530 741.According to the latter document, the layers of felt can be interleavedbetween the layers of fiber fabric in the core of a preform, or they canbe added to the faces of the core in order to constitute surface layersof a preform that are designed to be eliminated in machining operationsthat take place during and/or after densification.

Preforms can be densified by chemical vapor infiltration or by using aliquid, both of which techniques are well known. Chemical vaporinfiltration consists in placing the preforms that are to be densifiedin an enclosure into which a matrix-precursor gas is admitted that,under controlled conditions of temperature and pressure, diffuses withinthe preform and forms a deposit of matrix material on the fibers byreaction between its own components or by decomposition. When depositiontakes place preferentially in the surface pores of the preform, tendingto close them prematurely, it can be necessary to proceed with one ormore intermediate surface-machining or “descaling” operations in orderto recover surface pores and allow densification to proceed to the coreof each preform.

Densification by means of a liquid consists in impregnating a preformwith a matrix precursor in the liquid state, e.g. a resin, and thentransforming the precursor, generally by heat treatment. Severalconsecutive impregnation cycles can be necessary in order to achieve thedesired degree of densification. It is also possible to combine thetechniques of chemical vapor infiltration and of liquid impregnation.

Compared with metal disks, brake disks made of thermostructuralcomposite material, and in particular of C—C composite material, providea considerable saving in mass while providing excellent tribologicalproperties and low wear. They are also well adapted to the severeconditions of use encountered in airplanes and in F1 racing cars.

Extending the use of thermostructural composite brake disks to othertypes of vehicle, such as trains, heavy trucks, coaches, utilityvehicles, or private cars is being slowed down specifically because ofparticular problems encountered in such use.

Thus, tests performed by the Applicant on a top-of-range private carwith C—C composite brake disks made using a method analogous to thatused for manufacturing airplane brake disks have demonstrated that theycan sometimes give rise to undesirable vibration, and to braking torquethat can be irregular. In those brake disks, the preforms were made byneedling together layers of base texture, which texture was made up of aplurality of unidirectional sheets of cables disposed at differentangles (e.g. three sheets at 0°, +60°, and −60° C.) that were themselvespreneedled together. It is likely that the use of that base fabric givesrise to irregular wear of the friction faces of the disks that come intocontact with the brake pads, which phenomenon gets worse over thelifetime of the brake disks and generates vibration.

An object of the present invention is to provide a method of preparingfiber preforms that enables brake disks to be made out of compositematerial that do not present those drawbacks.

A particular object of the present invention is to provide such a methodenabling brake disks to be made of composite material that are suitablefor use on industrial or private motor vehicles without generatingundesirable vibration and regardless of braking conditions, while alsodelivering braking torque that is regular and without any wear that isabnormally fast.

Another object of the present invention is to obtain such performance ata cost price that is compatible with the brake disks being used inmass-produced industrial or private motor vehicles.

Such objects can be achieved by a method of the type comprisingsuperposing and bonding together fiber layers comprising structurallayers formed at least in part out of continuous filaments and out of atleast one felt layer, in which method structural fiber layers are usedto form at least a first preform portion that is to constitute the fiberreinforcement of the core of the brake disk, while the or each preformportion that is to constitute the fiber reinforcement of a frictionportion of the brake disk is constituted by a felt, at least in itsportion adjacent to the friction face.

The term “structural fiber layer formed at least in part out ofcontinuous filaments” is used herein to mean a layer that is woven,braided, or knitted out of continuous threads, themselves made ofcontinuous or discontinuous fibers, or a layer constituted by a sheet ofunidirectional, continuous cables, twisted strands, or threads, thecables, strands, or threads, themselves being constituted by continuousor discontinuous fibers, or else a layer constituted by a plurality ofsuch sheets superposed in different directions and bonded together, e.g.by preneedling, or indeed such a fiber layer associated with a thin webof fibers to which it is bonded, e.g. by light needling. Such structuralfiber layers are used to constitute a preform portion that is suitablefor conferring to the core of the brake disks the mechanical propertieswhich are required for transmitting braking forces without rupturing ordamaging the disks, in particular where the core is mechanically linkedto the member with which the disk is bound in rotation. The structuralfiber layers can be placed flat, parallel to the faces of the disks, orthey can be wound around the axis of the disks. If they are wound, theportion of the preform corresponding to the core of the disk can beobtained by cutting slices from a sleeve obtained by rolling up a stripof structural fiber fabric on a mandrel to form layers that aresuperposed on one another.

The felt forming, at least a part of the or each portion of the preformthat is to constitute the fiber reinforcement of a friction portion ofthe brake disk is in the form of at least one relatively thick layerhaving fibers at a low volume density, preferably less than 20%, wherefiber volume density is the fraction of the apparent volume of the feltthat is actually occupied by the fibers. The term “relatively thick feltlayer” is used herein to mean a felt which, in the prepared preform, isof a thickness that is not less than about 1 mm. After densification,the major portion of the friction lining in the vicinity of the frictionface is constituted by the composite material matrix. Typically, thefriction lining in the vicinity of the friction face has 10% to 15% byvolume of fibers, 65% to 75% by volume of matrix, and 15% to 20% byvolume of residual open pores.

By having a preform of this structure, and in particular by having feltpresent in the vicinity of the friction face, no undesirable vibrationappears during braking, contrary to that which has been observed withbrake disks in which the preform is constituted by needled structuralfiber layers, even in the friction portions. Such vibration can be dueto wear of the friction face that becomes irregular in the long term.The presence in the friction portion of felt, i.e. of non-oriented shortfibers, instead of structural fiber layers, and also the occupation ofthe majority fraction by the matrix, give rise to smaller anisotropy andless rigidity, thus avoiding the appearance of wear irregularities, orpromoting attenuation thereof.

In addition, it has been observed that the braking torque is remarkablyregular. Furthermore, the performance obtained is just as good in a wetenvironment as it is in a dry environment.

It can be envisaged to use felt not only for the or each portion of thepreform that corresponds to a friction portion of a disk, but also toform preferably thin layers that are interleaved between structuralfiber layers in the first portion of the preform that corresponds to thecore of the disk. When structural fiber layers are disposed parallel tothe faces of the disk, this contributes to imparting a certain amount offlexibility to the disk in the axial direction and increases itscapacity to absorb vibration.

The layers constituting the first portion of the preform correspondingto the core of the disk are preferably bonded together by needling. Thefelt constituting at least in part the or each portion of the preformcorresponding to a friction portion of the disk can be formed as asingle layer or as a plurality of superposed layers that are likewiseadvantageously bonded together by needling. Bonding between the felt andthe first portion of the preform can also be performed by needling. Itshould be observed that under such circumstances the felt must beneedled without being compressed in such a way as to increase the fibervolume density above the desired maximum.

An annular brake disk preform can be made from plane fiber layers eitherby superposing and bonding together fiber layers that are precut to anannular shape, or else by superposing and bonding together optionallycircular fiber layers without any center holes, and subsequently cuttingthe preform through all of the superposed and bonded together fiberlayers. It is also possible to make the portion of the preform thatcorresponds to the core of the disk by winding a fiber fabric assuperposed layers which are bonded together, while the or each portionof the preform corresponding to a friction portion of the disk is madeby superposing and bonding together fiber layers that are plane.

According to another aspect, the invention also provides a method ofmanufacturing brake disks out of composite material by densifyingpreforms prepared in the manner given above.

Advantageously, to manufacture an assembly comprising both a centralrotor brake disk having two opposite friction faces and also two endstator brake disks having one friction face each, e.g. for an industrialvehicle disk brake (heavy truck or coach), four substantially identicalcomponent preforms are made, each having a first portion correspondingto a core portion and a second portion corresponding to a frictionportion, the preforms are densified, and the rotor disk is obtained byputting two densified preforms together via their faces opposite fromtheir friction faces. This means that rotor disks and stator disksbecome different only after they have been densified. It is alsopossible to envisage assembling two component preforms prior todensification in order to obtain a rotor disk preform, in which caserotor disks and stator disks become different after the preforms havebeen prepared, but before densification.

According to yet another aspect, the invention provides disk brakesmanufactured from preforms prepared in the manner given above.

Implementations to the invention are described below by way ofnon-limiting indication. Reference is made to the accompanying drawings,in which:

FIG. 1 shows the successive steps in making an annular preform for adisk brake in an implementation of the method of the invention;

FIG. 2 shows the successive steps in making an annular preform for adisk brake in a variant of the method of FIG. 1;

FIG. 3 shows the successive steps in making a portion of a disk brakepreform in another implementation of the method of the invention;

FIG. 4 is a graph in which the curves show how braking torque varies asa function of time during braking when using disks obtained inaccordance with the invention and prior art disks;

FIG. 5 is a bar chart showing wear as measured during braking when usingbrake disks obtained in accordance with the invention and a prior artbrake disk; and

FIG. 6 shows the successive steps in making brake disks for a heavytruck, implementing the method of the invention.

The description below relates to preparing preforms out of carbon fiberor of carbon precursor for the purpose of manufacturing brake disks outof C—C composite material. Nevertheless, it should be observed that theinvention is applicable to disk brakes made of composite material otherthan C—C, in particular composite material having reinforcing fibersand/or a matrix that is, at least in part, made out of a ceramic, e.g.at least in part out of SiC, or out of a silicided C—C compositematerial.

An annular preform for a disk brake having two friction faces, e.g. adisk brake designed to co-operate with brake pads in a motor vehiclebrake, such as a mass-produced private car, can be made as follows (FIG.1).

The starting material used for the portion of the preform thatcorresponds to the core of the disk is a base fabric 10 made from fibersof carbon or a carbon precursor, which precursor could be, for example,pre-oxidized polyacrylonitrile (PAN), pitch, rayon, or a phenolcompound. When the preform is made from carbon precursor fibers, theprecursor is transformed by heat treatment, preferably after the preformhas been prepared and before it is densified. It will be observed thatthe preform that can be obtained with carbon fibers from a plurality ofdifferent precursors.

The base fabric 10 is made at least in part out of continuous elementsthat form a multidirectional two-dimensional fabric. The fabric may bewoven, braided, knitted, a unidirectional sheet, or as in the exampleshown diagrammatically, a superposition of a plurality of unidirectionalsheets of threads, cables, or twisted strands. The sheets are superposedwith different directions and they are assembled together by lightneedling. By way of example, the base fabric could be made up of threeunidirectional sheets disposed respectively at 0°, +60°, and −60° C.relative to an axis of the fabric. The base fabric may optionally befinished off with a thin web of fibers preneedled onto the fabric.

A first annular preform portion for a brake disk is made by stackingplies 12 while flat of the base fabric 10 and by bonding them togetherby needling. A plurality of thin felt layers 14 can be interleavedbetween each pair of plies 12. The term “thin felt layer” is used hereinto mean a layer of felt having density per unit area of less than 500g/m², e.g. lying in the range 200 g/m³ to 300 g/m³, and a fiber densityof less than 20%, e.g. lying in the range 7% to 14% when in the relaxedstate (prior to the compression that is due to needling). Needling isperformed by means of a needling head 20 in the form of a needle board,with the plies being placed on a support 22 which is covered by a basefelt 24 in which the needles can penetrate without being damaged. Thelength of the needle board corresponds substantially to the radialdistance between the inner circumference and the outer circumference ofthe annular preform portion that is to be made. Each time a new ply 12or layer 14 is deposited, an annular needling pass is performed. To thisend, e.g. as described in document FR-A-2 626 294, one full revolutionis performed between the needling head 20 and the support 22 about theaxis of the preform portion, and during this revolution, a predeterminednumber of needling strokes is performed by causing the needles topenetrate into the preform portion that is being prepared,perpendicularly to the surface thereof. Rotation can be obtained bydriving the needling head or the support 22 about the axis of thepreform. On each pass, the depth of needling can be maintainedsubstantially constant or it can be variable, e.g. increasing slightly,by lowering the support 22 step by step as the preform portion is builtup. Once a thickness has been reached that corresponds substantially tothe thickness of the core of the brake disk that is to be made, aplurality of finishing needling passes can be performed after the lastply has been needled, so as to obtain needling density per unit volumethat is substantially constant. Methods of preparing preforms withsubstantially constant needling densities per unit volume are describedin documents FR-A-2 584 106 and FR-A-2 726 013.

The preform portion 30 obtained in this way is finished off on each ofits faces by a layer of felt of thickness that is selected as a functionof the thickness required for the friction portions of the disk that isto be made. Thus, with the preform portion 30 being kept in place, afelt layer 16 is placed on the top face thereof and is bonded thereto byperforming an annular needling pass in the above-described manner. Thepreform portion 30 fitted with its felt layer 16 is removed from thesupport 20 and the base felt 24, is turned over, and then put back intoplace so as to make it possible to dispose and needle another felt layer18 on the other face, in the same manner as the layer 16.

In a variant, each felt layer 16, 18 can be replaced by two or morelayers needled in succession onto the preform portion 30. It is alsopossible to use one or more felt layers to form a portion only of thethickness of the preform portions that correspond to the frictionportions of the disk, with the remainder being formed by plies analogousto the plies 12, for example. In which case, the felt layer(s) is/aredisposed adjacent to the outside face (the friction face). In all cases,each preform portion that corresponds to a friction portion of the diskhas at least one relatively thick felt layer. The term “thick feltlayer” is used herein to designate a layer having mass per unit areathat is greater than 500 g/m², e.g. lying in the range 600 g/m² to 800g/m², with a fiber density less than 20%, e.g. lying in the range 10% to15% in the relaxed state. This corresponds substantially to a thicknessafter needling of not less than 1 mm, or to a thickness in the relaxedstate of several millimeters, e.g. not less than 3 mm.

The preform portion 30 with the felt layers 16 and 18 is cut out bymeans of a punch so as to obtain an annular preform 32 for a brake diskhaving a first portion 32 ₁ corresponding to the core of the disk andformed by the needled together plies 12 and layers 14, and two lateralportions 32 ₂ and 32 ₃ corresponding to the friction portions of thedisk, formed by the felt layers 16 and 18.

Above, it is envisaged that the preform is cut out to its annular shapeafter it has been needled. In a variant, as shown in FIG. 2, it ispossible to cut out the plies 12 and the felt layers 14, 16, and 18 soas to give them the desired annular shape prior to needling. Theneedling operation can be performed in a manner analogous to thatdescribed above, i.e. in successive annular needling passes. It is thenpreferable for the plies and layers of felt being needled to be held inplace by means of tooling formed at least by a central hub 26 projectingaxially from the support 22 carrying the annular base felt 24.

After forming a first preform portion 32, by needling annular plies 12together with optional interposed annular thin felt layers 14, and afteradding annular layers 16 and 18 on the faces of the preform portion 321and bonding them thereto by needling so as to form the two preformportions 32 ₂ and 32 ₃, a brake disk preform 32 is obtained similar tothat described above.

C—C composite brake disks 42 are made from preforms 32 prepared asdescribed above by subjecting the preforms to an operation ofdensification by means of a carbon matrix, where appropriate aftercarbonization heat treatment if the preforms are made from carbonprecursor fibers. In well known manner, densification is performed bychemical vapor infiltration or by a liquid technique.

The densified performs are then machined to give the disks 42 theirfinal dimensions and to form the notches that are necessary for linkingthem to the member with which they are fast in rotation.

The above description envisages forming the annular preform by stackingplies of fiber fabric and layers of felt flat parallel to the faces ofthe disk.

In a variant, it is possible for the portion of the preform thatcorresponds to the core of the disk to make use of a spirally orhelically wound strip of cloth optionally associated with a strip ofthin felt, wound flat in superposed layers around a central hub, withthe strip being needled as it is wound. A method of this type isdescribed in the French patent application filed under the No. 95 14000. In similar manner, for the or each portion of the preformcorresponding to a friction portion of the disk, it is possible to use afelt strip that is wound to form superposed layers that are needledtogether.

In yet another variant, as shown in FIG. 3, the portion of the preformcorresponding to the core of the disk is obtained from a strip 50 offiber fabric wound in superposed layers on a mandrel. The strip 50 is astructural fabric, e.g. a woven cloth, optionally associated with a thinfelt strip.

The strip 50 is needled substantially at the location where it issuperposed on the layers that have already been wound. Needling isperformed by means of a needle board 60 which extends across the widthof the strip 50. By way of example, the mandrel 62 is a rotary mandrelfitted with a base layer 64 into which the needles can penetrate whileneedling the first layers. After one full revolution, the mandrel 62 islowered relative to the needles so as to have a needling depth that issubstantially constant, or that can be varied slightly in controlledmanner. When the necessary number of layers 52 has been formed,finishing needling passes can be performed. A method of manufacturingcylindrical preforms with constant needling density per unit volume isdescribed in document FR-A-1 584 107.

In a variant, it will be observed that the strip can be wound by makingtangential contact with a roller, the mandrel being stationary andfitted with perforations in register with the needles, thereby making itpossible to omit a base covering.

The resulting needled sleeve 70 is sliced in radial planes to formannular preform portions 72 ₁ each corresponding to a brake disk core.

A preform portion corresponding to a brake disk friction portion andcomprising at least one felt layer is then needled onto one or each faceof the preforms 72 ₁. The installation of FIG. 2 can be used for thispurpose, advantageously associating it with a peripheral cylindricalwall 28 which, in co-operation with the central hub 26, ensures that thepreform portion 72 ₁ is guided.

EXAMPLE 1

C—C composite front brake disks for private vehicles of the “MercedesE600” type were made as follows, using the method of FIG. 1.

The base fiber fabric used for the preform portion corresponding to thecore of the disk was constituted by a laminate made up of threeunidirectional sheets of pre-oxidized polyacrylonitrile (PAN) fibershaving a mass per unit area of about 1000 g/m², superposed in threedifferent directions (0°, +60 °, −60° C.), and preneedled with a thinfelt having mass per unit area of about 300 g/m². A plurality of basefiber fabric plies were superposed while being needled togetherpreferably so as to obtain constant needling density per unit volume, asdescribed in document FR-A-2 584 106 or document FR-A-2 726 013, until athickness of about 22 mm was obtained.

On each face of the preform portion obtained in that way, eight feltlayers having a mass per unit area of about 800 g/m² were successivelysuperposed and needled so as to obtain a thickness of about 10 mm. Asbefore, each felt layer was needled so as to obtain needling atsubstantially constant density per unit volume.

An annular preform having inside and outside diameters respectivelyequal to 420 mm and to 180 mm was then cut out and subjected tocarbonizing treatment so as to transform the preoxidized PAN intocarbon, with the preform optionally being held in shape by tooling.

The preform was densified with a pyrolytic carbon matrix by chemicalvapor infiltration.

Disks obtained in that way were tested together with prior art C—Ccomposite material brake pads.

A first braking test was performed in the dry using a brake disk D1 andbrake pads obtained as mentioned above. Braking torque was measuredcontinuously throughout the braking required to cause speed to pass from200 km/h to 0. Curve A in FIG. 4 shows variation in torque measured as afunction of time. By way of comparison, comparative tests were performedusing the usual brake for the vehicle under consideration (cast ironbrake disk D2) and using a prior art C—C composite brake disk D3. Theprior art brake disk D3 differs from the disk D1 in that its preform wasmade entirely by needling together plies of the base texture 10 of FIG.1, without incorporating any felt, whether in its friction faces or inits core. Curves B and C in FIG. 4 show how measured torque varied as afunction of time when using the disks D2 and D3.

It will be observed that the time required for braking with C—Ccomposite disks was 2 seconds (s) shorter than that obtained when usingthe cast iron disks (8 s down to about 6 s), however the disk D3 gaverise to instabilities in braking torque which gave rise to large amountsof vibration. These variations in braking torque were considerablysmaller with the disk D1, and the improvement in torque stability (ratiobetween torque variation Δt and torque t) being more than 60%. It shouldalso be observed that the stability S in the coefficient of friction μwas greatly improved, where S is the ratio of the difference between themaximum value μ_(max) and the minimum value μ_(min) as measured duringthe test and as divided by the computed mean value μ_(mean):[S=(μ_(max)−μ_(min))/μ_(mean)] When using disks D1 of the invention, thevalue of S is 0.23 for braking in the dry and 0.39 for braking in thewet, whereas it is respectively 0.46 for braking in the dry and 0.61 forbraking in the wet when using the prior art disks D3. In addition,braking torque, and thus braking effectiveness, increases duringbraking, which is not the case with the disk D3.

A second series of dry braking tests was performed with disks mounted onthe front axle of the vehicle in question of the “Mercedes E600” type.Wear on the left and right disks was measured respectively for the disksD1 and the disks D3. FIG. 5 shows the measured wear in terms of loss ofthickness for the disks during high energy braking from 250 km/h to 100km/h. Remarkably, the wear measured on the disks D1 obtained inaccordance with the present invention was about half the wear measuredwith prior art C—C composite disks.

EXAMPLE 2

An assembly comprising a rotor brake disk and two stator brake disks fora heavy truck disk brake was made as follows (FIG. 6).

Identical component annular preforms 32 a, 32 b, 32 c, and 32 d wereprepared using the method of FIG. 1, as follows.

Each perform comprised a first portion corresponding to a portion of thecore of a disk, formed by superposing and needling together layers of abase fiber fabric, e.g. identical to that used in FIG. 1. On one of thefaces of that preform portion there were superposed and needled layersof felt to form a second preform portion corresponding to a diskfriction portion.

After carbonization, the preforms 32 a, 32 b, 32 c, and 32 d weredensified by a carbon matrix as described in Example 1.

Two stator disks 42 a and 42 b were thus obtained each having a core anda friction face corresponding to densified preforms 32 a and 32 b, andtwo rotor half-disks 42 c and 42 d were obtained for making up a rotordisk by being placed adjacent each other via their faces opposite fromtheir friction faces. The stator disks were half the thickness of therotor disks, which is entirely acceptable for the application underconsideration.

The disks were machined so as to bring them to their final dimensionsand form notches enabling them to be mounted. Thus, the rotor half-disks42 c and 42 d were provided throughout their entire thickness with axialnotches 44 c and 44 d extending from their inside faces for the purposeof co-operating with corresponding ribs on a shaft constrained to rotatewith a heavy truck wheel. The rotor half-disks can be assembled togetherdirectly by being mounted on the shaft, e.g. by being mutually clampedtogether via their hubs. The stator disks 42 a and 42 b were providedwith radial notches 44 a and 44 b extending from their innercircumferences and formed in a fraction only of the thickness of eachdisk starting from its rear face opposite from its friction face. Thenotches 44 a and 44 b were designed to co-operate with keys fortransmitting braking forces to the chassis of the heavy truck. Thismethod of providing mechanical linkage with the disk core makes itpossible for the core portion of the preform to be obtained by winding astrip in superposed layers extending perpendicularly to the faces of thedisk, as in the embodiment of FIG. 3. There is no risk of thetransmitted forces delaminating the preform, i.e. causing its layers toseparate from one another, as would be the case if forces weretransmitted from notches formed in the outer periphery of the core ofthe disk, and across the entire thickness thereof.

The implementation of FIG. 6 is particularly advantageous in that itmakes it possible to use standard preforms. In a variant, it may beobserved that the assembly required for obtaining the rotor disk can beperformed prior to densifying the original preforms. Naturally, theoption of preparing each kind of disk preform separately is notexcluded.

What is claimed is:
 1. A method of manufacturing a brake disk made of acomposite material having a fiber reinforcement densified by a matrix,the brake disk comprising a core portion and at least one frictionportion adjacent to the core portion and having an outer friction face,the method comprising the steps of: preparing a fiber preform by forminga first fiber preform portion that is to constitute the fiberreinforcement of the core portion by superposing and bonding together byneedling fiber layers comprising structural layers formed at least inpart out of continuous filaments, and by forming at least one secondfiber preform portion that is to constitute a fiber reinforcement of afriction portion of the disk, said second preform portion beingconstituted by at least one felt layer bonded to the first fiber preformportion by needling, densifying said fiber preform with a matrix, andmachining said densified fiber preform to final dimensions of the brakedisk, whereby a brake disk is obtained having a fiber reinforcementincluding structural layers in its core portion and having a fiberreinforcement constituted by felt adjacent the friction face of its atleast one friction portion.
 2. A method according to claim 1,characterized in that felt layers are interposed between said structuralfiber layers of said first preform portion.
 3. A method according toclaim 1, characterized in that said structural layers are formed byturns of a helical cloth wound flat in superposed layers.
 4. A methodaccording to claim 1, characterized in that the forming of the firstpreform portion includes forming a cylindrical sleeve by winding a fiberfabric strip into superposed layers about a mandrel and cutting thesleeve into slices along radial planes to obtain a plurality of firstpreform portions.
 5. A method according to claim 1, characterized inthat said at least one felt layer constituting the at least one secondpreform portion has a fiber volume density of less than 20%.
 6. A methodof manufacturing a composite material braking assembly comprising acentral rotor brake disk having a core portion and two friction portionsadjacent to the core portion on either side thereof and each having afriction face, and two stator disks coaxially arranged with said rotordisk on either side thereof, each stator disk having a core portion andone friction portion adjacent to the core portion and having a frictionface facing a respective friction face of the rotor disk, said disksbeing in a composite material having a fiber reinforcement densified bya matrix, said method comprising the steps of: preparing foursubstantially identical fiber preforms, each by forming a first fiberpreform portion that is to constitute the fiber reinforcement of atleast a part of a core portion of a disk by superposing and bondingtogether by needling fiber layers comprising structural layers formed atleast in part out of continuous filaments, and by forming a second fiberpreform portion that is to constitute a fiber reinforcement of afriction portion of a disk, said second preform portion beingconstituted by at least one felt layer bonded to the first fiber preformportion by needling, densifying the four fiber preforms with a matrix,putting two of the densified fiber preforms together via their firstpreform portions so as to obtain a densified preform for a rotor diskhaving two friction portions, and machining said two densified preformsto the final dimensions of a rotor disk, and machining each of the twoother densified preforms to the final dimensions of a stator disk,whereby a braking assembly is obtained with a rotor disk and two statordisks each having a fiber reinforcement including structural layers inits core portion and having a fiber reinforcement constituted by feltadjacent the friction face of its friction portion(s).
 7. A methodaccording to claim 6, characterized in that said step of putting twofiber preforms together via their first preform portions is carried outprior said step of densifying the fiber preforms with a matrix.
 8. Amethod according to claim 6, characterized in that the final machiningof each stator disk comprises forming notches that extend radially, andover a depth smaller than the thickness of the core portion of the disk,from a rear face of the disk opposite from its friction face.
 9. Amethod according to claim 8, characterized in that the forming of eachfirst preform portion includes forming a cylindrical sleeve by winding afiber fabric strip into superposed layers about a mandrel.
 10. A methodaccording to claim 7, characterized in that the final machining of eachstator disk comprises forming notches that extend radially, and over adepth smaller than the thickness of the core portion of the disk, from arear face of the disk opposite from its friction face.
 11. A methodaccording to claim 10, characterized in that the forming of each firstpreform portion includes forming a cylindrical sleeve by winding a fiberfabric strip into superposed layers about a mandrel.
 12. A brake disk ina composite material having a fiber reinforcement densified by a matrix,the disk comprising a core portion and at least one friction portionadjacent the core portion and having an outer friction face, wherein thefiber reinforcement of the core portion of the disk comprises structuralfiber layers formed at least in part by continuous filaments, and thefiber reinforcement of the at least one friction portion is constitutedof at least one felt layer, at least in its part adjacent to thefriction face, said structural layers and at least one felt layer of thefiber reinforcement being needled together.
 13. A brake disk accordingto claim 12, characterized in that said felt layer has a fiber volumedensity of less than 20%.
 14. A brake disk according to claim 12,characterized in that said at least one friction portion of said diskcomprises, by volume, 10% to 15% fibers, 65% to 75% matrix, and 15% to20% residual pores.
 15. A brake disk according to claim 12,characterized in that said fiber reinforcement of the core portion ofthe disk has felt layers interleaved between structural fiber layers.16. A brake disk according to claim 12, characterized in that said fiberreinforcement of the core portion of the disk comprises structural fiberlayers parallel to the faces of the disk.
 17. A brake disk according toclaim 12, characterized in that said fiber reinforcement of the coreportion of the disk comprises structural fiber layers perpendicular tothe faces of the disk.
 18. A brake disk according to claim 12,characterized in that said fiber reinforcement and the matrix are madeof carbon.
 19. Braking apparatus for a private car, characterized inthat it comprises at least one brake disk according to claim 12,co-operating with brake pads.
 20. Braking apparatus for a heavy roadvehicle, characterized in that it comprises at least one assembly of twostator disks and one rotor disk according to claim
 12. 21. Brakingapparatus according to claim 20, characterized in that each stator diskhas radial notches formed over a fraction of the thickness of the diskin its face opposite to its friction face.